218 



ZOOLOGY 



SECT. 



Fir:. 841. Premaxillpe of Sargus, 

 showing teeth. (After Owen.) 



In a large majority of species the teeth are small, conical, and 

 recurved, suitable for preventing the struggling prey from slipping 

 out of the mouth, but quite unfitted for either tearing or crushing. 

 In some Fishes, such as the Pike, the teeth are hinged backwards 

 so as to offer no resistance to the passage of the prey towards the 

 gullet, but effectually barring any movement in the other direc- 

 tion. In many deep-sea Fishes (Fig. 834) the teeth are of immense 

 size and constitute a very formidable armature to the jaws. 

 Many instances occur in which there is a marked differentiation 

 of the teeth, those in the front of the jaws (Fig. 841) being pointed 



or chisel-edged, and adapted for 

 seizing, while the back teeth have 

 spherical surfaces adapted for 

 crushing. In the Wrasses (Fig. 

 830, B) strong crushing teeth are 

 developed on the pharyngeal bones. 

 In the Globe-fishes the teeth are 

 apparently reduced to one or two 

 in each jaw, but each " tooth in 

 this case really consists of numer- 

 ous calcified plates fused together. 

 The teeth may be either simply 

 imbedded in the mucous mem- 

 brane so as to be detached when the 



bones are macerated or boiled, or they may be implanted in sockets 

 of the bone, or ankylosed to it. They are formed of some variety 

 of dentine, and are often capped with enamel. Their succession 

 is perpetual, i.e. injured or worn-out teeth are replaced at all ages. 

 In some species the enteric canal shows little differentiation into 

 regions, but, as a rule, gullet, stomach, duodenum, ileum, and 

 rectum are more or less clearly distinguishable. The stomach is 

 generally V-shaped, but its cardiac region may be prolonged into 

 a blind pouch : it is often very distensible, allowing some of the 

 deep-sea Teleostei to swallow Fishes as large as themselves. In the 

 Globe-fishes the animal can inflate the gullet with air, when it floats 

 upside down on the surface of the water. The Ganoids have a s/>/W 

 valve in the intestine, which is very well developed in Polypterus 

 and the Sturgeon, vestigial in Lepidosteus (Fig. 843, y. r.) and 

 Amia : it is absent in all Teleostei, except possibly in Chirocentrus, 

 one of the Physostomi. The liver is usually large ; a pancreas 

 may be present as a compact gland, as in Elasmobranchs, or may 

 be widely diffused between the layers of the mesentery. Pyloric 

 cceca are commonly present, and vary in number from a single one 

 to two hundred. The anus is always distinct from, and in front 

 of, the urino-genital aperture. 



Respiratory organs.- -The gills are usually comb-like, as in 

 the Trout, the branchial filaments being free, owing to the atrophy 



